1. Technical Field
The present invention relates to a laser apparatus. In particular, the present invention relates to a laser apparatus including a λ/4-shifted DFB (Distributed Feedback) fiber laser and a ring resonator.
2. Related Art
One type of lasers that oscillate in a single longitudinal mode is a DFB laser. A DFB laser includes, as an optical feedback path, a diffraction grating formed for example by periodically modulating the refraction index of an optical waveguide.
FIG. 1 is a partial sectional diagram schematically showing the structure of a semiconductor DFB laser 90. As shown in the drawing, in the semiconductor DFB laser 90, an InGaAsP active layer 93 and a p-InGaAsP waveguide layer 92 are sequentially stacked inside a buffer layer 94 on an n-InP substrate 96. In addition, a diffraction grating 91 is formed in the boundary between the InGaAsP active layer 93 and the p-InGaAsP waveguide layer 92.
As shown in “1.5-μm λ/4-shifted InGaAsP/InP DFB Lasers,” J. of Lightwave Technol., vol. 5, no. 11, pp. 1564-1573 (1987), the semiconductor DFB laser 90 equipped with a diffraction grating that includes a phase shift corresponding to λ/4 is able to stably oscillate in a single longitudinal mode. Because of its single longitudinal mode oscillation as well as small size and low cost, the semiconductor DFB laser 90 has been widely used as a light source for optical communication.
FIG. 2 schematically shows a structure of a DFB fiber laser 40, which is formed by applying the technology of the λ/4-shifted lasers mentioned above to optical fibers. As shown in the drawing, the gain medium of the DFB fiber laser 40 is a rare-earth-element-doped core between a non-output end 42 and an output end 44. In addition, this core includes a diffraction grating 41 that includes a phase shift 43 corresponding to λ/4, and contains a distribution feedback structure by means of FBG (Fiber Bragg Grating). This DFB fiber laser 40 has a narrower linewidth compared to a semiconductor laser, and has an advantage such as easy connection with an optical fiber on the transmission path.
FIG. 3 schematically shows a structure of a DFB ring laser 100 that includes a ring resonator 130. As shown in this drawing, the DFB ring laser 100 includes a DFB structure 120 for single longitudinal mode operation, a ring resonator 130 formed by an annular optical waveguide 132, and an optical coupler 140 for taking out a part of light propagated in the ring resonator 130 to outside as an output. In addition, the DFB structure 120 is supplied with excitation light via a waveguide 110.
The DFB ring laser 100 is able to decrease an effective resonator loss by feeding back a laser oscillation signal to the DFB structure 120 via the ring resonator 130, thereby decreasing the laser oscillation threshold value condition, which is disclosed in the U.S. Pat. No. 6,272,165, and “Distributed-Feedback Ring All-Fiber Laser,” OSA TOPS on Advanced Solid-State Lasers, vol. 1, pp. 291-295 (1996). In addition, the ring resonator 130 has a high Q value, which helps narrow the linewidth of an outputted laser.
For use as a light source for a fiber laser optical communication or the like, an optical fiber doped with a high concentration of erbium is already known for achieving a high laser output. A high concentration of erbium, however, will lead to a serious intensity fluctuation such as a pulse oscillation, due to energy transfer incident to generation of an erbium-ion pair. For the purpose of restraining the generation of such an ion pair, a method is known and has been implemented, to codope aluminum into the core of the optical fiber.
Also for the same purpose as mentioned above, it is also known to form a laser using an optical fiber in which the core is made of phosphate glass that contains erbium and ytterbium, and is surrounded by a ring-shaped silica glass clad that contains germanium and boron. This technique is disclosed for example in “High Performance Single Frequency Fiber Grating-Based Erbium:Ytterbium-Codoped Fiber Lasers,” J. of Lightware. Technolo., vol. 16., no. 1, pp. 114-118 (1998), and “Efficient single-frequency fiber lasers with novel photosensitive Er/Yb optical fibers,” Opt. Lett. Vol. 22, no. 10, pp. 694-696 (1997)
After the result of the aforementioned attempts, an optical fiber having a phosphate glass core turned out to achieve a higher laser output, with disadvantages of incurring higher manufacturing cost due to its complicated structure, as well as lack of long-term reliability attributable to the hygroscopic property of phosphate glass. This makes it difficult to put down to practice such an optical fiber having a phosphate glass core.
As opposed to this, a high-output laser apparatus obtained by doping a high concentration of erbium into a silica optical fiber is known to operate stably for a long period of time. The present invention therefore aims to enable a laser apparatus to be of high output by doping of a high concentration of erbium, as well as to emit further less noise by codoping of aluminum.